Author Topic: Need help with understanding part of the circuit in Agilent PSU to repair it  (Read 2191 times)

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Offline ddcc

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Also I don't understand what it's doing on schematic for main board when marking R370 is on daughter board(see attachments)

No, you're looking at the wrong board. Although the component references can overlap between the main board and control board, there's a different one on the main board. You should refer to the component locator on PDF pg. 208, which shows them on the far right. It seems that the two pairs of resistors are on the bottom of the main board, probably close to the output terminals.
 

Offline Arek_R

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Oh right, sorry :D
Ok so I made more measurements.
To avoid possiblity of me screwing up calculations for these summing ampifiers or misunderstanding something, I just referenced to working CH2, which shows 0.585V at pin #14 of U100, and also it reacts accordingly to changing OVP preset, reaches about 2.56V when OVP set to 1V, seems right.
Where faulty CH1 shows -7.41V at pin #14 of U136, and it doesn't change when I change OVP preset.
I ignored sense amplifiers because it is used when you use remote sense, but anyway I checked at least R370 and R371 and of course they're fine.

So now I'm waiting for your comment on these readings, meanwhile I will also check resistors around the suspect.

EDIT: Checked resitors, haven't found anything...
« Last Edit: April 30, 2018, 10:30:53 pm by Arek_R »
 

Offline ddcc

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Are you sure you're measuring the right pin? Pin 14 of U100 (channel 2) corresponds to pin 1 of U136 (channel 1), not pin 14.

The remote sense functionality is always being used, because for local operation you put the jumper wires on the back of the unit from S+ to V+ and S- to V-. That's how it ensures that the local output voltage is correct.

At the moment, it sounds like at least one of your opamps are bad, although it's not quite clear which since they're all connected in circuit. To save some time, you could probably just order a couple of TL074CDs, and try replacing U25 and U136, one at a time.

Edit: Have you checked the +/- 15V rails for U136 and U25? They're unlikely to be bad since the other channel works, but might as well double check.
« Last Edit: April 30, 2018, 11:50:45 pm by ddcc »
 

Offline Arek_R

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Yeah, dot marks first pin, this is 14pin package so on opposite side.

I don't use any jumper wires, is that bad? xD

Ordered 3x of them right now,, mf by ti.

Only on U25 and it's ok.
 

Offline ddcc

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Hmm, you're right, there's a local bypass somewhere so the jumpers aren't necessary when remote sense is disabled. I don't know where that is on the schematic though, so your problem may not be where I thought it was. You could check the node labeled 3V connected to CR145, but I don't fully understand this portion of the schematic. It seems that the optoisolated CV_REF1 signal is fed into pin 5 of U136, and the output on pin 7 ends up being sampled by the microcontroller at CV1, but I don't see a connection to OUT1 (unless it's through the V1 net?). It's also a bit confusing because channels 1 and 2 don't quite mirror one another; there's an optoisolator on OVP_REF2.1 to OVP_REF2.2, but not for OVP_REF1.

I'm saying that you seem to be measuring the wrong pin for channel 1. On channel, measuring pin 14 of U100 is correct, because it's the output of the summing amplifier. But for channel 1, the output of the summing amplifier is on pin 1 of U136. So the -7.41V that you see on pin 14 of U136 is normal, because it's a voltage follower connected to a fixed voltage divider. So you should repeat your measurements on pin 1 of U136, and see if the voltage changes when you modify the OVP and the output voltage.

 

Offline Arek_R

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DAC/ADC circuitry is on potential of CH1, so it's connected directly to it, where signals/references for CH2 are fed into analog optoisolators.

EDIT: You can see this vid, dude explains a little bit what's going on, also I had to replace same resistor as him(+ 9 other resistors)
 

Offline Arek_R

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AAAnd it's fixed.
Sorry for not replying but I had a lot on my head and then holidays.
Today I decided to replace suspects, started with U25(this quad opamp) and it was it, PSU is now fully functional  ;D
Now it only needs calibration using proper DMM, calibrated with my normie UT139 for now.
https://drive.google.com/file/d/1ttRAYV4fs23mXUCKImqGOAUTbtdvzzd3/preview
« Last Edit: May 09, 2018, 05:48:07 pm by Arek_R »
 

Offline ddcc

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Nice, congratulations! You may want to consider looking into the DMMCheck Plus, a new production run was restarted recently.
 

Offline Arek_R

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Looks nice!
I actually have broken 34401A on way to me, will fix it, send it for proper calibration and then calibrate this PSU(and other stuff).
Also 34401A is actually recommended in manual of this PSU :D
 


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